Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see
Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to
www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

Prerequisites for PPP over Frame Relay

Before you can configure PPP over Frame Relay, Frame Relay must be enabled on the router using the encapsulationframe-relaycommand.

Information About PPP over Frame Relay

Overview of PPP over Frame Relay

The PPP over Frame Relay feature allows a router to establish end-to-end Point-to-Point Protocol (PPP) sessions over Frame Relay. IP datagrams are transported over the PPP link using RFC 1973 compliant Frame Relay framing. This feature is useful for remote users running PPP to access their Frame Relay corporate networks as shown in the figure below.

Figure 1. PPP over Frame Relay Scenario

The figure below shows a connectivity scenario using the Cisco 90i D4 channel card, which is capable of supporting Integrated Services Digital Network (ISDN) Digital Service Loop (DSL), PPP, or Frame Relay, which connects to an Internet Service Provider (ISP) or corporate network.

Figure 2. PPP over Frame Relay Frame Format

PPP over Frame Relay is compliant with the functionality and encapsulation specifications as outlined in RFC 1973. The frame format is shown in the figure below.

Figure 3. PPP over Frame Relay Frame Format

A PPP connection is established over the Frame Relay PVC. The PPP session does not occur unless the associated Frame Relay PVC is in an "active" state. The Frame Relay VC can coexist with other circuits using different Frame Relay encapsulation methods, such as RFC 1490 and Cisco proprietary, over the same Frame Relay link. There can be multiple PPP-in-Frame-Relay circuits existing on one Frame Relay link.

One PPP connection resides on one virtual access interface, which is internally created from a virtual template interface. A virtual access interface is cloned from a virtual template interface. The virtual access interfaces is coexistent with the creation of the Frame Relay circuit when the corresponding DLCI is configured. One virtual template interface, containing all the necessary PPP and network protocol information, is shared by multiple virtual access interfaces. Hardware compression and fancy queuing algorithms, such as weighted fair queuing, custom queuing, and priority queuing, are not applied to virtual access interfaces. Once a Frame Relay circuit is established using PPP over Frame Relay, all incoming and outgoing packets on this circuit are under RFC 1973 PPP-in-Frame-Relay encapsulation compliance until this DLCI is removed from the configuration.

The breakdown of the Frame Relay frame format components is listed in the table below.

Table 1 PPP Frame Relay Format Descriptions

Field

Description

Flag

A single byte that indicates the beginning or end of a frame.

Address

A two-byte field that indicates the logical connection that maps to the physical channel; the DLCI.

Control

A single byte that calls for transmission of user data. PPP over Frame Relay uses a value of 0X03, which indicates that the frame is an unnumbered information (UI) frame.

NLPID

Network layer protocol ID, which is a single byte that uniquely identifies a PPP packet to Frame Relay.

PPP protocol

Identifies the PPP packet type.

Benefits

Allows end-to-end PPP sessions over Frame Relay.

Supports the 90i IDSL Channel Unit that supports both Frame Relay and PPP on an ISDN DSL.

How to Configure PPP over Frame Relay

Enabling PPP over Frame Relay

Perform the following task to configure the physical interface that carries the PPP session and links to the appropriate virtual template interface.

Before You Begin

After you configure the Cisco router or access server for Frame Relay encapsulation, you must configure the physical interface with the PVC and apply a virtual template with PPP encapsulation to the DLCI that it applies to.

By default, the encapsulation type for a virtual template interface is PPP encapsulation; therefore, the encapsulationppp command will not show up when viewing the running configuration on the router.

Example PPP over Frame Relay DCE

The following example configures a router to act as a data communications equipment (DCE) device. Typically, a router is configured as a DCE if connecting directly to another router or if connected to a 90i D4 channel unit, which is connected to a telco channel bank. The three commands required for this type of configuration are frame-relayswitching, frame-relayintf-typedce, and frame-relayroute commands.

MIBs

RFCs

RFC

Title

RFC 1973

PPP in Frame Relay

Technical Assistance

Description

Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

Feature Information for PPP over Frame Relay

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to
www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

The following commands were introduced or modified:
debugframe-relayppp,
frame-relayinterface-dlci,
showframe-relaypvc.

Glossary

data-linkconnectionidentifier(DLCI)--A value that specifies a PVC or SVC in a Frame Relay network. In the basic Frame Relay specification, DLCIs are locally significant (connected devices might use different values to specify the same connection). In the LMI extended specification, DLCIs are globally significant (DLCIs specify individual end devices).

LinkControlProtocol(LCP)--A protocol that establishes, configures, and tests data link connections used by PPP.

permanentvirtualcircuit(PVC)--Virtual circuit that is permanently established. PVCs save bandwidth associated with circuit establishment and teardown in situations where certain virtual circuits must exist all the time.

virtualcircuit(VC)--A logical circuit created to ensure reliable communication between two network devices. A virtual circuit can be either permanent (a PVC) or switched (an SVC). Virtual circuits are used in Frame Relay and X.25.